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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

FATTY SUBSTANCES  (GENERAL METHODS)

142-6.

379

for neutralisation 17-3 c.c. of N/2-KOH, i.e., 17-3 x 0-02805 = 0-4852 gram
KOH, the acetyl acid value being therefore

485-2 _

3-402

After addition of a further quantity of 30 c.c. of N/2-KOH and boiling, 11-5
c.c. of N/2-HC1 were necessary to neutralise the excess of potash. The potash
consumed in the saponification of the acetyl compounds was hence 30-11-5 =
18-5 c.c., corresponding with 18-5 X 0-02805 = 0-5189 gram KOH ; for i gram
of acetylated acids the amount of KOH will be

518-9

3-402

= 152-5-

The acetyl number is therefore 152-5, and the acetyl saponification number,
142-6 + 152-5 =s 295-1.

The acetyl number is related to the quantities of hydroxylated acids and
higher alcohols in the fatty substances, these being especially large in castor
oil, grapeseed oil, waxes and blown or oxidised oils. It is consequently of
some importance for the identification of these fats and for the analysis of
waxes.

12. Iodine Number

This expresses the number of grams of iodine 100 grams of a fatty sub-
stance are capable of fixing under definite conditions. It may be determined
in the two following ways :

A. Hiibl's Method.—REAGENTS required:—

1.    Iodine   solution.      25   grams   of   iodine   are   dissolved   in    500
c.c. of 95% alcohol (puriss.) ;  in another 500 c.c. of the same alcohol, 30
grams of mercuric chloride are dissolved and the solution filtered, if neces-
sary.   The two solutions are stored separately in tightly stoppered bottles
in a cold dark place, equal volumes of them being mixed in the quantities
required for the number of tests to be made, about 48 hours before use.

2.  Potassium iodide solution.    10 grams  of potassium iodide (puriss.)
quite free from iodate are dissolved in 100 c.c. of water.

3.  Starch  solution,    i gram of  starch  is made  into a paste with a
little cold water and then poured into about 300 c.c. of boiling water, stirred
and left to settle * when cold, the clear supernatant liquid is poured off for
use as indicator.   Soluble starch also may be employed, this being prepared
by digesting potato starch with dilute hydrochloric acid (D 1-05) for a
week, then washing with water by decantation until the washing water
is free from hydrochloric acid and drying between filter-papers at a moderate
temperature ;   i gram of this starch is dissolved in 100 c.c. of hot water.

4.  Sodium   thiosulphate   solution.   25   grams   of   crystallised   sodium
thiosulphate    (puriss.)    are   dissolved   in   distilled   water   to   i   litre,
and the strength of the solution determined by Volhard's method, as follows :
20 c.c. of a solution containing 3-863 grams of pure potassium dichro-
mate per litre are shaken with 10 c.c. of the aqueous 10% potassium iodide
solution and 5 c.c. of hydrochloric acid (D i-io) ;   100-150 c.c. of water
are then added and the liberated iodine titrated with the sodium thio-
sulphate solution, a little of the starch paste being added towards the end
of the titration.    Since 20 c.c. of the above potassium dichromate solution
set free 0-2 gram of iodine from potassium iodide in presence of hydrochloriccess of